1. Field of the Invention
The present invention relates to a method and apparatus for detecting and/or imaging a voltage signal of at least one specific frequency at a measuring location of a test object utilizing a scanning particle or radiation microscope.
2. Description of the Prior Art
In testing and analyzing integrated circuits for faults, it is useful to compare the behavior of the integrated circuit in question with the rated behavior derived from a simulation of the correct operation of the circuit in order to locate an error or fault. For this purpose, among other tests, a check must be performed to determine whether specific internal periodic signals of different frequencies are present at a particular measuring location within the integrated circuit.
Methods for undertaking such a check which are known in the art may be classified generally into three basic methods. The method of so-called "voltage coding" is known from the article "Voltage Coding: Temporal Versus Spatial Frequencies", Lukianoff et al, Scanning Electron Microscopy/1975 (Part I), Proceedings of the Eighth Annular Scanning Electron Microscope Symposium, Chicago, IIT Research Institute, pages 465-471. The "voltage coding" method generates an image of the dynamic distribution of voltage signals of an integrated circuit on a television monitor. The "voltage coding" method enables a chronological allocation of the respective switching status of the various components and is therefore particularly suited for fast function testing of integrated circuits. The "voltage coding" method, however, has disadvantages due to the line frequency of the electron beam.
So-called "logic state mapping" is known from U.S. Pat. No. 4,223,220. The "logic state mapping" method generates an image of the dynamic distribution of voltage signals by means of a stroboscope effect. Compared to the "voltage coding" method, the "logic state mapping" method supplies a time resolution which is higher by several orders of magnitude given the same voltage resolution. The "logic state mapping" method also simplifies registration, because the images of the dynamic distribution of the voltage signals can be directly photographed from the picture screen of a scanning electron microscope. In the "voltage coding" method, however, registration of the dynamic distribution of voltage signals is only possible with a tape storage or with photographs taken from a television monitor.
A third known method is described in the article "Une Alternative Economique au Contraste de Potentiel Stroboscopique: Le Traitement du Signal d'Electrons Secondaires d'un Microscope a Balayage", Collin, appearing in Proc. of Journees d'Electronique 1983, "Testing Complex Integrated Circuits: A Challenge" published by the Swiss Federal Institute of Technology, Lausanne, Switzerland at pages 283-298. In this method detection of specific frequencies at a measuring location in an integrated circuit is executed by the use of a "lock-in" amplifier. The "lock-in" amplifier is employed to filter out a signal having the frequency in question from a voltage contrast signal acquired at a measuring location in the integrated circuit. The intensity of this signal is imaged as brightness fluctuation. In this known method, the electron beam is not pulsed.
This third known method has two disadvantages, the first being that "lock-in" amplifiers generally exhibit a highly limited output bandwidth in amplitude measurement (the input bandwidth of a "lock-in" amplifier is generally higher than the output bandwidth, so that the limitation of bandwidth at the output does not result in an enhancement of the sensitivity). The limited output bandwidth of a "lock-in" amplifier in the third known method requires long picture exposure times which may last several minutes (such as, for example, 3 through 20 minutes) and further causes a high load on the specimen making this method not well suited for routine use when seeking a signal having a specific frequency in an integrated circuit. The second disadvantage of the third known method is that the maximum operating frequency is limited by the limit frequency of the "lock-in" amplifier, and is also limited by the limit frequency of the signal chain of the scanning electron microscope which is employed. for an Ithaco "lock-in" amplifier type 391A, for example, the limit frequency is 200 kHz. For an ETEC Autoscan II scanning electron microscope, the limit frequency of the signal chain, which essentially consists of a detector, a photomultiplier and a pre-amplifier, is in the range of approximately 1 to 2 MHz. Such a scanning electron microscope is described, for example, in U.S. Pat. No. 4,277,679. Higher operating frequencies cannot be attained with this third known method so that under certain conditions the integrated circuits in question cannot be investigated at their operating frequency. For example, integrated bipolar circuits and numerous MOS circuits have operating frequencies higher than 2 MHz.
Thus the three methods known in the art can only be implemented with difficulty, and when implemented, permit a check of only a few locations or nodes of the integrated circuit and are further greatly limited in their range of operating frequencies, often making a check of an integrated circuit under normal operating conditions impossible. Such methods are furthermore very slow and presume that the frequency of the signal in question is known, or that a signal synchronous with the signal in question is externally accessible. If the frequency of the signal in question is not known, the search for such a signal becomes very tedious and complex. This is the case, for example, if the frequency of the signal in question is achieved by means of division by 255 instead of division by 256.